Your comments

This is a feature I would love to see added. I'm not an expert on diff tools, but I use WinMerge quite a bit - a compromise between its features and ST's design philosophies would be really nice to see.

I think it's important to keep ST as feature-minimal as possible, but if notepad++ has a built-in diff tool I think it's okay to add one ..

Features like this are best left to plugins. In my darker moments I want stuff like this, but then I remember that it makes me a lazy programmer, and it adds bloat to an otherwise, err...sublime...tool.